Electrical monitoring circuits are known which provide a warning signal, usually in the form of a warning light, indicating the failure of or a fault in a lamp. One such system, described in U.K. Patent No. 1,342,248, monitors the lighting circuits of a motor vehicle in order to detect any lamp failures which may occur. In that system the potential difference across a low value resistor inserted in series with each lamp is monitored and when the potential difference is zero a fault is indicated.
Another such system, also employing a low value series resistor as a detector and designed to overcome undesirable temperature-dependence characteristics of earlier systems, is described in U.K. Patent No. 2,034,948 B. Moreover, this system provides additional monitoring facilities to discriminate between lamp failure and faults due to the low value series resistor being open-circuit or a fuse failure. This latter patent also describes in detail various integrated circuit embodiments of such systems which can monitor groups of lamps including lamps which are required to operate only intermittently such as the vechicle's brake lights.
The known systems provide an indication of the failure of a vehicle's lamps during their use only and may be described as energised or `hot` monitoring systems; they are not capable of monitoring a lamp that is not energised, i.e. they do not have a de-energised or `cold` monitoring capability.
It is an object of the present invention to provide a means whereby the detection of a failure of the filament of a vehicle's lamp may be made whilst that lamp is de-energised or `cold`.
It has been suggested that `cold` monitoring could be achieved by providing a bleed resistor across the on/off switch for the lamps, such a resistor providing a small leak current through the lamp(s) and associated in-series resistors when the switch is open; the voltage across the in-series resistor could then be monitored in the same way as `hot` monitoring. However, the leak current must be very low so as to avoid undue draining of the battery and thus the voltage drop across the in-series resistor would be minute (e.g. 10 microvolts, which contrasts with a typical voltage drop during hot monitoring of approximately 20 millivolts).
A technique known as `auto-zeroing` has been known for several year to reduce amplifier (and comparator) errors to very low (unmeasurable) levels. This technique uses two operational amplifiers which alternate in serving the input signals; while one amplifier is performing the comparison (or amplification) function, the other is nulling itself and storing a correction term on a capacitor. The input is then transferred to the recently nulled amplifier and the previous amplifier corrects its errors. This cycle is repeated under a suitable controlling switching circuit. No-one has suggested applying such a technique to cold monitoring of vehicle bulbs but it is not a technique which is readily applicable to use in vehicles since auto-zeroing is highly sensitive to voltage transients and so would be unreliable in a vehicle where the voltage supplied by the battery can vary considerably depending on the load on the battery.
A further problem arises if the same comparator is to be used to perform both `cold` monitoring and `hot` monitoring since the input to the comparator in the former case would be a potential close to earth while the input in the latter case would be the voltage rating of the battery (usually 12 volts) and this would lead to great unreliability in operation of such a sensitive comparator.